Should I keep the same teams in training or mix them constantly?

What match problem are you trying to reproduce? If your U10s aren't building understanding, constant rotation might be preventing them from solving the tactical problems inherent in their pairings. In a 7v7 environment, I would keep the same teams together for the full session to allow those relationships to settle. Try a simple 7v7 game with a 'three-touch' constraint for the defending team only. This forces the team in possession to recognize when they have time to build play versus when they need to penetrate quickly, while giving the attacking team the stability of consistent teammates to execute those decisions. Observe how their movement off the ball improves when they start anticipating each other’s tendencies. You don't need to overcomplicate this with progressions; the consistency of the team itself is the only constraint you need to see better football.